The ever-increasing demand to process, store, and exchange information creates an unceasing driving force for high bandwidth, energy-efficient photonic technologies. In recent years, the need to develop photonic devices with extremely high energy efficiency to atto-joule/bit has been recognized. Silicon photonics has the potential to transform future optical interconnect systems by reducing the energy consumption and enhancing the bandwidth of existing electronic systems by orders of magnitude using Complementary Metal-Oxide-Semiconductor (CMOS) compatible fabrication processes. In addition to the application to optical interconnects, silicon photonic devices can also operate logic gates to conduct certain types of optical computation. However, the performance of silicon photonic devices is still limited by the diffraction limit and the relatively weak plasma dispersion effect. Although silicon has a relatively high refractive index, it can only shrink the wavelength inside the silicon waveguide proportionally to the scale of λ/n, roughly to 400˜600 nm. Further reduction of the device footprint requires exploiting surface plasmon polaritons, which are bound waves at the interface between a metal and a dielectric. The extremely strong light confinement of metal-insulator-metal waveguides has led to the demonstration of ultra-compact and high bandwidth plasmonic electro-optic (E-O) modulators. However, plasmonic structures and devices are very lossy and can only carry information over a very short distance, and hybrid plasmonic-dielectric waveguide integration must be used for real optical interconnects, which increases the complexity of design and fabrication.
A further constraint of silicon photonic devices is that the plasma dispersion effect induced by free carrier injection or depletion can only induce moderate refractive index perturbation. For example, for a typical depletion-based silicon photonic modulator with a moderate doping level of 2.5×1018 cm−3 in its active region, when completely depleted, the refractive index only changes by 0.06%. As a result, current Mach-Zehnder interferometer (MZI) silicon modulators require a long device length up to hundreds of micrometers to several millimeters to accumulate sufficient phase modulation. The large device footprint also leads to a large energy consumption of pico-joule/bit, which cannot meet the requirement of future photonic interconnects application. Compared with MZI modulators, resonator-based E-O modulators occupy a much smaller footprint and achieve significantly higher energy efficiency. To date, various ultra-compact silicon micro-ring resonators, micro-disk resonators and photonic crystal nano-cavities have been used in optical interconnect systems, achieving high performance in modulation speed, compactness, and energy efficiency. However, resonator-based modulators have an intrinsic trade-off between energy efficiency and optical bandwidth. For practical devices, thermal control with integrated heaters and temperature sensors are often used to obtain stable performance, but with the sacrifice of additional energy consumption and footprint.
In addition, existing transparent conductive oxide-based E-O modulators based on straight silicon waveguide or plasmonic slot waveguide use electrically-induced optical absorption from the integrated MOS capacitor. The phase change induced by the real part of the permittivity of transparent conductive oxide materials, although accompanied by the imaginary part of the index change, does not contribute to any E-O modulation. Therefore, a relatively long modulation length (a few microns) is required to induce sufficient optical absorption. Moreover, these modulators require the presence of metal gates for strong plasmonic light confinement and electronic signal conductance, which introduces relatively high optical loss even at the transparent state.
Accordingly, there is a need in the art for E-O modulators which are energy efficient and have increased bandwidth while occupying a smaller footprint.